cpufreq: interactive: Fix compiler warning for hvt

Compiler warns about hvt variable in
cpufreq_interactive_speedchange_task() being used without
initialization. Initialize it to ~0ULL.

Note that in reality, this won't happen because when governor_enabled
is true, these two conditions are guaranteed:
1) policy->cpus won't be empty, and
2) target_freq won't be 0.
Otherwise, it indicates more serious issues in the system.

With these two conditions, hvt will be overwritten by at least
one CPU's local_hvttime.

Change-Id: I4378393ed811674f25d54852c296ee5ff407e7e3
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>